A lot of events in SoCal are handled by 911 ambulance companies (in riverside county it's pretty much only AMR who does events). Symons Event Safety in Devore, CA is an event company. They handle some events in riverside county, San Bernardino county, LA/OC county, and several other counties. You have to be 21 years old in order to drive their ambulance (as of 3 years ago).
I know there is another company in the riverside/San Bernardino area that does events but I don't remember the name right now.
Many of the local raceways have contracts with the 911 ambulance company that covers the area.
Morongo Casino in Cabazon hires EMTs, they just hired a couple of people a month ago. I have heard of other casinos hiring EMTs but once again I am not sure which ones.